Maxima gives $-{{x\,\log \left(\sin ^2\left(2\,x\right)+\cos ^2\left(2\,x\right)+ 2\,\cos \left(2\,x\right)+1\right)+2\,i\,x\,{\rm atan2}\left(\sin \left(2\,x\right) , \cos \left(2\,x\right)+1\right)-i\,{\it li}_{2}( -e^{2\,i\,x})-i\,x^2}\over{2}}$, where ${\it li}_{2}$ is something called Spence's function. Unless it's possible to simplify Spence's function for this argument, I think the answer to your question is no. This is more appropriate for math.stackexchange.com than for mathoverflow, which is for research-level questions. You should learn to mark up your posts in LaTeX.
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Ben CrowellOct 2 '12 at 4:51

17

@Benjamin: I am less sure this is not research level, unless Picard-Vesiot is being taught at lower-levels than I'd imagined... I for one would be interested to learn of an argument that this function is not integrable in terms of elementary functions, rather than rely on Deep Thought to tell me "42"
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Yemon ChoiOct 2 '12 at 4:53

6

Hey, am I the only person here who wasn't taught how to immediately recognize rigorously which functions have elementary antiderivatives?
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Yemon ChoiOct 2 '12 at 4:54

1

Like Maxima, Maple evaluates this in terms of the dilogarithm function, or equivalently in terms of the non-elementary $\int dx/\log(x)$.
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Gerald EdgarOct 2 '12 at 11:31

9

Victor: I strongly believe that you should "unaccept" my answer and accept Peter Mueller's instead, as he has answered your question in full by appealing to the theorem of Liouville to which I referred.
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Benjamin DickmanMay 16 '13 at 0:59

4 Answers
4

Edit (6/14/14): I maintain that Peter Mueller's answer should be accepted, since he has essentially carried out the proof using Liouville's Theorem (mentioned below) to show that the integrand under question cannot be expressed in elementary functions. However, in addition to my previous unsuccessful attack, I thought I would also point to a full argument in the following source:

If you're going to resort to term-by-term integration of an infinite series, why not just take the power series for $x\tan x$ and integrate that?
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Gerry MyersonOct 3 '12 at 1:40

That's a good question. If you just check wolframalpha.com/input/?i=integral+xtanx you'll get an answer involving $Li_{2}(-e^{2ix})$. I think re-writing it as I did above is helpful for a few reasons. One, it lets you see rather quickly how that $Li_{2}$ pops up; it's somewhat surprising (to me) that $\log$ should rear its head at all in an attempt at integrating $x \tan x$. Two, I think it might help in evaluating specific examples, e.g. $\int_{0}^{\pi/4} x \tan x dx$. Three, I find it pedagogically helpful to see what goes wrong in attempting to find such an antiderivative.
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Benjamin DickmanOct 3 '12 at 2:34

Finding an anti-derivative of $x\tan x$ amounts to finding an anti-derivative of $f=\frac{x}{e^x+1}$. Consider the field $K=\mathbb C(x,e^x)$. Note that $K$ is closed under taking derivatives. If $f$ is elementary integrable, then Liouville's Theorem gives elements $u_i\in K$, $\gamma_i\in\mathbb C$, $v\in K$ with
\begin{equation}
\frac{x}{e^x+1}=\sum\gamma_i\frac{u_i'}{u_i}+v'.
\end{equation}
Consider the $u_i$ and $v$ as rational functions in $e^x$ with coeffcients in $\mathbb C(x)$. By the property of the logarithmic derivative we may assume that the $u_i$ are actually distinct irreducible monic polynomials with respect to $e^x$, or elements from $\mathbb C(x)$.

Looking at poles (with respect to the `variable' $e^x$) shows that at most one of the $u_i$ is $e^x+1$, and the other $u_i$'s are in $\mathbb C(x)$. Similarly, we see that $v\in\mathbb C(x)$. So there indeed must be one index $i$ with $u_i=e^x+1$. However, $\frac{x}{e^x+1}-\gamma_i\frac{u_i'}{u_i}=\frac{x}{e^x+1}-\gamma_i\frac{e^x}{e^x+1}$ isn't in $\mathbb C(x)$, a contradiction.

Remark: The argument given here is somewhat sketchy, some routine details need to be filled in, like that $u_i'$ and $u_i$, as polynomials in $e^x$, are relatively prime. A beautiful paper about Liouville's Theorem is Rosenlicht's article Integration in finite terms. My argument somewhat follows Rosenlicht's example of finding an anti-derivative of $f(x)e^{g(x)}$, where $f$ and $g$ are rational functions.

Thinking about how to simplify this kind of argument for the general masses... Is it possible to convert this kind of calculation into the computation of some "invariant" associated to $x\tan x$ that vanishes iff it has an elementary antiderivative?
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Igor KhavkineOct 2 '12 at 10:37

Well, this argument is beautiful -- I never came across such things before, that here complex analysis would be of help. I'm still to go through all things to understand it well, but it's very nice.
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VictorOct 2 '12 at 10:46